Optically based biomedical imaging techniques have advanced over the past decade due to factors including developments in laser technology, sophisticated reconstruction algorithms and imaging software originally developed for non-optical, tomographic imaging modes such as CT and MRI. Visible wavelengths are used for optical imaging of surface structures by means of endoscopy and microscopy.
Near infrared wavelengths (approx. 700-1000 nm) have been used in optical imaging of internal tissues, because near infrared radiation exhibits tissue penetration of up to 6-8 centimeters. See, e.g., Wyatt, 1997, “Cerebral oxygenation and haemodynamics in the fetus and newborn infant,” Phil. Trans. R. Soc. London B 352:701-706; Tromberg et al., 1997, “Non-invasive measurements of breast tissue optical properties using frequency-domain photo migration,” Phil. Trans. R. Soc. London B 352:661-667.
Advantages of near infrared imaging over other currently used clinical imaging techniques include the following: potential for simultaneous use of multiple, distinguishable probes (important in molecular imaging); high temporal resolution (important in functional imaging); high spatial resolution (important in in vivo microscopy); and safety (no ionizing radiation).
In near infrared fluorescence imaging, filtered light or a laser with a defined bandwidth is used as a source of excitation light. The excitation light travels through body tissues. When it encounters a near infrared fluorescent molecule (“contrast agent”), the excitation light is absorbed. The fluorescent molecule then emits light (fluorescence) spectrally distinguishable (slightly longer wavelength) from the excitation light. Despite good penetration of biological tissues by near infrared light, conventional near infrared fluorescence probes are subject to many of the same limitations encountered with other contrast agents, including low target/background ratios.